Specify theme context for images in Markdown

Users can now specify the theme an image is displayed for in Markdown. Appending #gh-dark-mode-only or #gh-light-mode-only to the end of an image url will define whether it's only shown to viewers using a light or a dark GitHub theme. For example:

Two screenshots of the same issue in dark and light color mode demonstrating the two fragment options by showing an optimized github logo for each color mode

The GitHub logo in the above screenshots uses the following Markdown to specify the theme context:

![GitHub-Mark-Light](https://user-images.githubusercontent.com/3369400/139447912-e0f43f33-6d9f-45f8-be46-2df5bbc91289.png#gh-dark-mode-only)![GitHub-Mark-Dark](https://user-images.githubusercontent.com/3369400/139448065-39a229ba-4b06-434b-bc67-616e2ed80c8f.png#gh-light-mode-only)